Alkylbenzenes are useful, for example, as intermediates in the production of various end products. As an embodiment of this invention, among the products which can be synthesized more efficiently using alkylbenzenes prepared in accordance with this invention are members of the family of compounds which comprise the 2-aryl propionic acid derivatives. Specifically, ibuprofen, a commercially successful, over-the-counter, analgesic can be synthesized using isobutyl benzene prepared as a raw material.
It has been known for decades that alkali metals, when reacted with alkylbenzenes, will displace benzylic hydrogens. The resulting alkylbenzene anion/alkali metal cation pair will undergo a reaction with olefins at certain temperatures to give alkylation products in which some or all saturated benzylic carbon atoms are alkylated in such a way as to replace some or all of the benzylic hydrogen atoms on a carbon atom with one aliphatic chain per benzylic hydrogen atom. Such reactions can yield a variety of products, depending on the number of saturated benzylic carbon atoms and the number of hydrogen atoms on a given benzylic carbon atom. In the commercial production of alkylbenzenes, a product of high purity is generally desired, and byproducts must be removed.
An especially serious problem with the present alkali metal catalyzed alkylation reaction is the fact that the alpha carbon of the said alkylbenzene anion can add to either carbon atom comprising the olefinic double bond, giving two alkylation products.
Moderation of reaction conditions has proven to be effective in eliminating multiple alkylations. However, inability to conveniently improve upon addition specificity has heretofore remained problematic.
The presence of isomers due to non-specific addition of the alkylbenzene to the olefinic double bond has commercial consequences. In particular, it can have a grave impact on the overall rate of production of isobutyl benzene. For example, with the current method of producing isobutyl benzene (IBB), large amounts of normal-butyl benzene (NBB) are formed. As the boiling point of IBB (171.degree. C.) is similar to the boiling point of NBB (183.degree. C.), distillation to high purity is greatly complicated due to the necessity of removing a byproduct with a similar boiling point. In fact, the time required to distill off enough NBB to give an IBB product of sufficient purity for the required applications can cause purification to be the rate-limiting step in the production of IBB.
In addition to isomer formation, another problem with present processes is that the alkali metal catalysts utilized in the alkylation reaction seem to exacerbate the formation of insoluble tarry byproducts. Analysis of this byproduct indicates that it is comprised of high molecular weight molecules, likely the result of an alkali metal catalyzed polymerization reaction.
Other byproducts are formed as well. These are generally soluble in the reaction mixture, and their presence may cause the reaction mixture to have a darkened color.
The formation of tars and other byproducts has serious commercial consequences, as illustrated in the large-scale production of IBB. A current commercial method for the production of IBB is carried out as a batch process using a sodium/potassium alloy. The catalyst composition is prepared in situ by charging to the reactor toluene, small amounts of tall oil and water, and an amount of NaK.sub.2 such that the mole ratio of NaK.sub.2 to toluene is about 0.024. The temperature is raised to around 190.degree. C. After about 15 minutes, propylene is fed to a reactor pressure of about 350 psig, and a NaK.sub.2 to propylene mole ratio of about 0.028. After roughly 4-6 hours, the reaction mixture is cooled to about 50.degree. C., and the reaction mixture is drained, and IBB is separated out. The process can be reiterated for successive batch production of IBB. Constant mechanical agitation is employed during the reaction in order to keep the catalyst and associated catalytic species emulsified; emulsification greatly increases the surface area on which benzyl potassium, the catalytically active complex in the alkylation reaction discussed above, can be formed, thus giving an enhanced reaction rate. It has been theorized that benzyl potassium coats each droplet of alloy, and that the alkylation occurs at the surface of the droplets. As the reaction proceeds, it is surmised that tar formed at the surface of a catalyst droplet accretes on the droplet, eventually encasing it. As the melting point of the tar is too high to be safely reached by the reactor, the accumulated tars inactivate large amounts of catalyst with each reaction run, making it necessary to recharge the reactor with catalyst in order to keep the reaction rate high. In addition, tar formation can shrink the reactor volume available for formation of product, giving diminishing yields of IBB at constant energy input. Also, frequent tar plugs in reactor lines often force a complete reactor cleaning, a drastic action which results in a waste of reactants and catalyst. Tar removal is complicated by the alkali metal core of each tar granule. Known methods must be conducted with extraordinary care, as the risk of uncontrolled reaction can be high; explosions are not unknown.
The formation of isomers, tars, and other byproducts has even further commercial consequences: a negative impact on catalyst and reactant utilization. Tar encased alkali metal has a greatly reduced ability to catalyze the formation of the alkylation product desired, and it is ultimately destroyed in the tar removal process. For example, a commercial process which produces 150,000 pounds of isobutyl benzene can be expected to lose over 0.25 million dollars due to unrecycled catalyst. Furthermore, reactants which could be used to form the desired product are discarded as byproducts. In the formation of isobutyl benzene, for instance, one mole of normal-butyl benzene is formed for every nine moles of isobutyl benzene. The formation of tars and other byproducts reduces reactant utilization to about fifty percent.
Schramm and Langlois (Journal of the American chemical Society, 1960, 82, 4912-4917) present a detailed study of the effect of different alkali metals on the yield of byproducts, particularly isomers due to non-selective addition at the olefinic double bond, at a wide range of temperatures. In trials in which potassium metal was used as a catalyst in the alkylation of toluene with propylene to produce IBB, Schramm, et al. observe a roughly temperature invariant value of nine for the alkylation product ratio of isobutyl benzene (IBB) to normal butyl benzene (NBB) over the temperature range of 107.degree. C. to 204.degree. C. In contrast, sodium catalyzes the alkylation in a temperature dependent manner, heavily favoring IBB at lower temperatures. An IBB to NBB ratio of about twelve is measured at 307.degree. C., monotonically increasing to around twenty-four at 204.degree. C. However, the Schramm and Langlois find that the data implies an activity for potassium that is at least an order of magnitude higher than that of sodium at all temperatures measured, specifically over the range of from 149.degree. C. to 204.degree. C. For example, at 149.degree. C., sodium is expected to have an activity which is roughly one one-hundredth the activity of potassium. The activities of both metals increase monotonically with increasing temperature.
An illustration of the above findings can be seen in the current commercial process. Despite the fact that sodium catalysis gives an IBB to NBB ratio of roughly 23 at 190.degree. C., Successive iterations of the process give product ratios, as defined above, of about nine, just as pure potassium gives in Schramm, et al. One might expect the sodium/potassium alloy to behave more like potassium due to sodium's relatively small activity at 190.degree. C.
It would be of great importance if a way could be found of producing desired alkylbenzenes with greater selectivity and without formation of tars and other byproducts. If such could be accomplished while enabling recycle of catalyst residues from run to run, the state of the art would be vastly improved.